Osmotic distillation is a technique for concentrating a liquid by placing the liquid to be concentrated and a second liquid of higher osmotic pressure on opposite sides of a microporous hydrophobic membrane. The differences in osmotic pressure result in solvent flow from the liquid of relatively low osmotic pressure to the liquid of higher osmotic pressure. See U.S. Pat. No. 4,781,837 to Lefebvre.
The successful practice of osmotic distillation for the concentration of liquid foodstuffs, beverages, and aqueous solutions of bioactive substances requires the interposition between such a liquid and a concentrated aqueous salt solution of a microporous, hydrophobic membrane which prevents entry of either liquid phase into the pores of the membrane (and consequent mixing of the two phases), while permitting unimpeded transfer of water vapor and other volatile components from one liquid phase to the other. This same process is also useful for the selective removal from an aqueous feed stream of a volatile solute, such as the dealcoholization of an alcoholic beverage such as wine or beer.
Many liquids whose concentration or other treatment by this means is desired contain nonvolatile surface active solutes which may concentrate and precipitate, or adsorb upon, the surface of such a membrane, rendering its surface, and the surfaces of the pores within it, hydrophilic and water-wettable. In that event, liquid penetration into and through the membrane pores will ensue, with consequent unacceptable contamination or loss of the product. A means for preventing this wet-out and penetration of the membrane upon contact with such feed streams is thus essential.
This limitation can be avoided by interposing, between the feed stream and the microporous, hydrophobic membrane, an extremely thin layer of a nonporous hydrogel of relatively high water content. Ideally, such a layer is freely permeable to water and other solutes (both volatile and non-volatile) which are present in the feed stream, but at the low pressures employed in osmotic distillation, will not pass liquid water, or any non-volatile solutes contained therein. At the interface between the hydrogel layer and the underlying microporous membrane, the only components present in the feed stream which can freely enter and penetrate the membrane pores are the vapors of water and other volatile solutes. All non-volatile solutes are constrained to the hydrogel, and must return by diffusion into the contacting feed liquid. It is thus impossible for the underlying microporous membrane to be contacted with a liquid phase containing surface active components, which can wet-out and penetrate the membrane pores.
The use of such hydrogel layers has been reported outside the context of osmotic distillation. See U.S. Pat. No. 4,419,187 and U.S. Pat. No. 4,419,242, both to Cheng et al.
Such a hydrogel layer must, of course, completely cover the exposed feed-side surface of the membrane, and be completely free of liquid-penetrable defects. On the other hand, this layer should be as thin as practicable, in order to minimize its resistance to transport of all volatile components. In addition, this layer must be strongly adherent to the underlying membrane so that it will resist detachment and/or damage by the fluid moving over it. Moreover, the layer should be of sufficient chemical and thermal stability to avoid its deterioration in extended contact with feed solutions of varying composition, as well as in contact with conventional cleaning and sterilizing media and conditions required in food and pharmaceutical processing. Finally, the layer must be capable of facile formation and placement on the underlying membrane by means which are adaptable to economic large scale membrane fabrication and lamination techniques.
Most hydrogels are formed from water-soluble polymers which are dissolved in water in relatively high concentration (of the order of 10% by weight or more), and then subjected to secondary crosslinkage while in solution by, for example, chemical reaction with crosslinking reagents, or by thermal conversion. Such materials and methods are hardly ideal for rapidly producing very thin layers on membranes at reasonable cost, for several reasons.
First, aqueous solutions of such polymers are usually of very high viscosity, so that their deployment as very thin films on either sheet- or hollow-fiber microporous membranes is exceedingly difficult. While use of more dilute, less viscous solutions would circumvent this problem, it would also be necessary to concentrate the solution so applied by an intermediate drying step, prior to treatment to insolubilize the polymer.
Further, such solutions have relatively high surface tensions, and do not spread on and wet hydrophobic surfaces. Consequently, it is virtually impossible to deposit thin, uniform, defect-free coatings of such polymers on these surfaces. While addition of a wetting agent to the solution might aid deposition of such a uniform thin film, it would have the disadvantage of also promoting "wicking" of the solution into the pores of the underlying membrane, thereby greatly impairing the function of the membrane laminate.
Moreover, such crosslinked hydrogel films normally exhibit poor adhesion to hydrophobic substrates. The use of corona-discharge, plasma-treatment, or surface-oxidation of the substrate membrane prior to coating can sometimes be used to ameliorate this problem. However, this is not only a costly additional operation, but often seriously weakens and deteriorates the membrane.
There is a need for materials and methods for producing such laminates which circumvent the foregoing limitations, and which afford the production of laminates (and of membrane modules containing such laminates) in a convenient and economic manner. This invention is directed to this important end.